68 THE LARCH CANKER 
As he points out, finer larch trees are found in mixed 
woods than generally occur in pure plantations, even when 
these are free from canker. This is partly due to the fact 
that ‘ when mixed sparingly with deep-rooting or compact- 
rooting trees, the larch-roots can spread without meeting 
much opposition, while their more rapid stem-growth gives 
them a lead over other species from the first ’. Also the soil 
under mixed woods is maintained in a state more generally 
suitable to tree-growth than in pure coniferous plantations. 
One resulf which we gain from the considerations of 
contributory causes is that the conditions which are favour- 
able to the restriction of canker are in general identical 
with those that encourage the active growth of the tree. 
Keep the trees growing vigorously and canker is not so 
likely to become epidemic. The explanation of this may be 
sought in the nature of the struggle between the parasite 
and the host. Given an uninterrupted tissue of living cells, 
the fungus can push forward, killing the cells in advance of 
it by its secretions and itself growing into them. But the 
tree resists this progress by cork layers, which we may 
liken to a series of trench systems, each holding up the 
enemy for a time, and perhaps entirely preventing his 
advance. When the fungus grows down from a branch, 
the first trench it has to capture is the cork layer normally 
made at the base of a dying branch. If this is passed, fresh 
cork layers are made in the tissues of the tree, and it often 
happens that the first of these prevents the canker from 
spreading farther. If this fails, more and more cork layers 
are formed, and so the struggle progresses—purely offensive 
on the one hand, and purcly defensive on the other. For- 
tunately the advance of the fungus through living tissue is 
very slow, especially in a tree which is healthy and fast- 
growing, so that such a tree has time to make a fresh cork 
layer (or, to revert to the simile of war, it can complete new 
earthworks before they are reached by the advancing 
enemy), and the better and more thoroughly this layer is 
made the less chance the fungus has of passing it. Not 
infrequently trees which are growing strongly show a number 
